Glass loads in panels, handrails and guards

[Rio]

RE CBC Sec. 2407.1.1..... When it says a safety factor of 4 will be used "The panels and their support system shall be designed to withstand the loads specified in Section 1607.7. A safety factor of four shall be used." As a load of 50plf is required, 200 plf would satisfy this part. For the handrail will the same amount apply or will the amount required for a point load (see below) also be multiplied by a factor of four, resulting in a load of 800 pounds?

Also, this is for a residential project and there is an exception to the rule of 50 plf. It says "For one- and two-family dwellings, only the single concentrated load required by Sec. 1607.1.1 shall be applied." Does this mean the 50 plf can be ignored?

CBC Sec. 1607.7.1.1 "Handrail assemblies and guards shall be able to resist a single concentrated load of 200 pounds (0.89KN), applied in any direction at any point along the top, etc."

Thanks for any clarifications...................

 

[Francis Vineyard]

Rio, when glass is used in this manner it is an engineered assembly. The 2006 IBC Commentary;

This section requires that the support system for glass guard or handrail assemblies be designed based on a safety factor of 4. Nominally identical panes of glass inherently have a wide variation in strength. The safety factor of 4 is used in the design to minimize the likelihood that breakage will occur below the design loads. It is not intended that an in-place glass guard or handrail system be tested for or capable of withstanding four times the design load.

Sounds like a very nice home.

 

[tbz]

RE CBC Sec. 2407.1.1..... When it says a safety factor of 4 will be used "The panels and their support system shall be designed to withstand the loads specified in Section 1607.7. A safety factor of four shall be used." As a load of 50plf is required, 200 plf would satisfy this part. For the handrail will the same amount apply or will the amount required for a point load (see below) also be multiplied by a factor of four, resulting in a load of 800 pounds? Also, this is for a residential project and there is an exception to the rule of 50 plf. It says "For one- and two-family dwellings, only the single concentrated load required by Sec. 1607.1.1 shall be applied." Does this mean the 50 plf can be ignored?

CBC Sec. 1607.7.1.1 "Handrail assemblies and guards shall be able to resist a single concentrated load of 200 pounds (0.89KN), applied in any direction at any point along the top, etc."

Thanks for any clarifications...................

Good morning Rio,

This one has been close to my heart for a long time, I submitted a change this last code cycle only to have it turned down.

The code is specific in stating that when glass is used within a guard or handrail the guard or handrail shall resist the load. The glass call out notation requires the 4 times safety factor be used in section 1607.7....

Many of engineers have argued with me that ASEC7 (it might be ASCE-7 don't have book open) any way, they argue that the glass is the only item that requires the safety factor, not the entire frame work, however when applying that load to the glass it needs to transfer to the building to stay in place.

1/2" glass at around 600lbs bends like 1/2" plywood when using the glass as the support system with a top and bottom holding channel. You will never get the load to full if this is a straight section of glass or a glass guard with no top.

Thus the glass bends below 42" before the load is reached.

The question becomes how much deflection is allowed?

This is an article on that portion that I find very helpful when explaining my point on this.

http://pieforensic.com/articles/articles/deflections-in-deck-and-balcony-railings.html

The main issue is the safety factor applies to the load, not to the glass.

As to the exception, that is CA. fix in the IBC since they don't use the IRC, the IRC only requires the 200lb point load at the top in any direction, which is why I always get in to heated discussions about adding design references to guard posts in the code, the codes does not state posts must resist, just that the top of the guard must resist.

 

[Rio]

Thanks for the feedback. Along these same lines does anyone know if a railing composed of glass panels on an elevated deck requires a cap rail?

 

[Mark K]

The safety factor of 4 is used when designing the glass elements. If steel elements were to be used the factor of safety would be closer to 1.7. The high factor of safety is needed because there is significant variation in the strength of the glass and because when overload occurs the glass fails in a brittle manner.

Large railing deflections may be an irritant but I do not believe that they necessarily indicate an unsafe condition. One of the things that I have observed is that when the railing is flexible people stay away from the railing thus increasing safety.

The linked article by Mr. Blackmer talks a lot about serviceability but fails to find a code provision that establishes deflection limits for glass railings. He makes an argument as to what he thinks is right but from a regulatory perspective we are interested in what the code requires not on what one might feel is appropriate..

Fatigue is caused by the variation in stress. Depending on the configuration you can have high deflection with low stress or high stress with low deflection. Fatigue is also associated with many cycles of stress. The lower the stress the more cycles will be needed. I believe that it would be a very unusual household for fatigue to be a factor with a glass railing since most loads on the glass railing will be significantly less than 50 plf.

A continuous railing at the top of glass panels shares the load among several panels and provides some redundancy if an individual panel fails... Thus I would encourage one be installed.

 

[RJJ]

Good topic! Good posts!

 

[tbz]

I am not sure if the CBC is following the 2009 modified or the 2006 modified IBC, but prior to 2009, I have to dig up a copy, but there was a printed formal or non-formal interp from the ICC that was done stating that a top was required on glass guards, no exposed edge allowed.

A 2009 code change allowed for certain types of glass to definitively be allowed without question. However, there are those that have taken the postion with 2006 and prior IBC additions, that the code gives the option that the guard must have either a top or handrail attached, but at minimum needs atleast one of them. Thus allowing exposed glass edge tops when a handrail was attached to the guard system.

The IBC commentary in 2006 is some what clear, but MARK I would differ that the steel would not need the safety factor of 4 designer also.

2407.1.1 Loads. reads

2407.1.1 Loads. The panel and their SUPPORT SYSTEM shall be DESIGNED to withstand the loads specified in Section 1607.7. A safety factor of four shall be used. Commentary continues to say: This section requires that the SUPPORT SYSTEM for glass guard and handrail assemblies be designed based on a safety factor of 4......

California does not use the IRC so the table in section 301 for guards and it's (I) note, which limits the safety factor of 4 to the glass only does not comply in CA, IMO.

Both code commentarys are specific that they need not pass a phyisical test of 4 times, just the standard test limits, but if one goes to look for ES information, AC-273 (I beleive is the right number) they require all items in the testing be done to a 2.5 safety factor, thus not code, but out there.

Where the IRC differs from the IBC is in not specifically noting anything other than adding a safety factor of 4 to the glass in table 301 & then specifying that the glass is located in a hazardish location for tempering/safety glazing.

So IMO if under 2009 modified, I would say exposed glass top edge is allowed, 800 point load under 2009 up for interp by AHJ, but COMMENTARY says DESIGNED, NOT TESTED.